Escalators that travel along a path which is curved in plan view are generally described in the prior art. Such escalators are generally graceful and impressive, but present many complexities due to the third dimension involved in their construction. It will be appreciated that the outer step rollers on a curved escalator will have to move faster than the inner step rollers since the distance they travel between landings is greater than the distance traveled by the inner rollers. This problem has been solved in the prior art in U.S. Pat. No. 4,730,717 granted Mar. 15, 1988 to K. Sugita by providing two different diameter drive and reversal sprockets for the inner and outer step chains, both mounted on a common shaft. The outer sprocket is larger than the inner sprocket, thus its teeth move faster than the teeth of the inner sprocket. This causes the outer step chain to move faster than the inner step chain. A complication that results from such a solution is that the steps must tip when they pass over the drive and return sprockets. When the steps have intermeshing cleats on the risers and adjacent tread edges, provisions must be made to avoid jamming of the cleats when the steps are thus tipped.
Curved escalators also experience considerable side thrust due to the path of travel, whereby the steps are pulled toward the center of the curved path along which they travel. This problem can be overcome by the inclusion of side thrust rollers on the chains which may engage the outside surface of the inner or outer step tracks, as shown in U.S. Pat. No. 4,739,870 granted Apr. 26, 1988 to R. Saito. The curved escalators, like linear escalators, will have one set of sprockets which are powered, and thus are drive sprockets, and will have at the opposite landing a set of idler sprockets which simply guide the step chains around the reversal, and are not directly powered. It is conventional as shown in U.S. Pat. No. 3,419,127 granted Dec. 31, 1968 to H.R. Yost, which is incorporated herein by reference, to provide a tension carriage for the idler sprockets, whereby the idler sprockets will be biased on a sliding carriage in a direction away from the drive sprockets. This ensures a constant tension on the step chains and thus prevents step chain sag. Since the step chains can stretch due to the tensioned idler sprockets, the tracks over which the step rollers move must also have some axial flexibility. This problem has been solved by providing a splice in the tracks as shown in U.S. Pat. No. 4,739,870 and others. A complicating factor which is not addressed in the prior art relates to the side thrust rollers, which will also have to negotiate a track splice. The splice disclosed in the 4,739,870 patent cannot be quietly and smoothly negotiated by the curved escalator's side thrust rollers.
One more complicating factor arising in connection with the curved escalator relates to the track path of the inner step rollers as the inner track passes from its position parallel with the outer track wherein the step treads are horizontal, to the inner sprockets, wherein the inner track is not parallel with the outer track, and the step treads are tipped. U.S. Pat. Nos. 4,746,000 granted May 24, 1988 to H. Nakatani discloses an inner track that dips along a rectilinear path from its horizontal position down toward the inner sprocket. This type of arrangement is not desirable for several reasons. First, when the step rollers move from the horizontal part of the track into the rectilinear dip zone, they will make distinct noise, which is not desirable. Secondly, at the idler sprocket end of the escalator, the track splice must be on a horizontal part of the track since the tension carriage slides horizontally. The part of the track that moves will be mounted on the tension carriage, and the rest will be mounted on the escalator truss. An arrangement which places as little of the track as possible on the tension carriage is the most desirable arrangement. With the rectilinear dip zone shown in the prior art, the track splice must be placed before the dip zone, thus the entire dip zone and part of the horizontal track zone are all mounted on the tension carriage, and not on the escalator truss. This will unduly stress the tension carriage.